Generally, graphite is a material having a typical layered, planar structure where two-dimensional graphene sheets stack. In each layer, carbon atoms are arranged in a hexagonal lattice. Graphene is a typical single layer sheet in which three carbon atoms are bonded together by sp2 hybrid orbital bonds arranged in a hexagonal crystal lattice.
In graphite, the bond between carbon atoms within one layer of graphite, i.e., graphene, is a strong covalent bond, but, the one between adjacent graphene layers is the Van der Waals bond, which is extremely weak compared to the covalent bond.
Graphene is one layer of graphite, i.e., a single layer of graphite. In graphite as the bond between adjacent graphene layers is very weak as described above, there can exist graphene which has a very thin two-dimensional structure of a thickness of about 4Å.
Graphene has unusual properties different from common materials. The most noticeable property found in graphene is that electrons flow like zero mass where electrons transfer in graphene. Thus electrons flow with the same velocity as light transfers i.e., with a velocity of light in vacuum. In addition, graphene shows the feature of a half-integer quantum hall effect, abnormal relative to electron and hole.
Also, currently known electron mobility of graphene has a high value of about 20,000 to 50,000 cm2/Vs. For example, carbon nanotubes are in a family similar to graphene since carbon nanotubes have very low yield where they are subject to refining after synthesis even when the synthesis is made using inexpensive material and their final product is expensive. Meanwhile graphene has a benefit of very inexpensive cost. Also, in the case of single walled carbon nanotubes, according to their chiral property and diameter, metal and semiconductor features vary and even if they have the same semiconductor feature, they have bandgaps that are all different. Thus to utilize a specific semiconductor property or metal property from a given single walled carbon nanotube, single walled carbon nanotubes each should be separated. However, it is known that this is very hard.
On the other hand, in the case of graphene, since its electrical property varies according to crystal directionality of graphene of a given thickness, a user can exhibit an electrical property into its selected direction and thus graphene has the merit that the user can easily design a device. Such features of graphene are very effectively available in carbon based electrical devices or carbon based electromagnetic devices.
Graphene has thus attracted much attention due to its useful and unusual properties as a substituent material for the next generation silicon and indium tin oxide.
Therefore, various methods for obtaining graphene have been continuously reported since 2004 and the methods generally includes mechanical exfoliation, chemical exfoliation, SiC crystal grown by thermal decomposition, exfoliation-reintercalation-expansion method, chemical vapor deposition and epitaxial synthesis, etc.
A mechanical exfoliation method utilizes the adhesive force of scotch tape, which comprises first attaching a cellophane tape to a graphite sample and detaching the cellophane tape from the graphite sample and then collecting graphene exfoliated from the graphite and attached on the surface of the cellophane tape. However, in the case of this mechanical exfoliation, the exfoliated graphene has the shape of torn paper and is thus not uniform. Further the size of the graphene is a mere level of micrometer and it is impossible to obtain a large size of graphene. As the final yield is extremely low, there is a problem that many samples are not suitable for necessary researches.
A chemical exfoliation method involves oxidizing graphite and crushing the graphite oxide via ultrasonic waves, etc., to prepare graphene oxide dispersed in an aqueous solution and then again reducing the graphene oxide by a reducing agent such as hydrazine to graphene. However, the graphene oxide is not completely reduced and only about 70% of graphene oxide is reduced, thus there still remains many defects in the graphene. Therefore, there is the problem that good physical and electrical properties specific to graphene get lowered.
A SiC crystal thermal decomposition method involves the principle that SiC is decomposed at its surface when a SiC single crystal is heated and then Si is removed and graphene is generated by the remaining Carbons. However, in the case of this thermal decomposition method, a SiC single crystal, which is used as a starting material, is extremely expensive and it is very hard to obtain large-scaled graphene.
An exfoliation-reintercalation-expanson method involves inserting fuming sulfuric acid into graphite, placing the resultant graphite on a furnace of high temperature and thus the sulfuric acid is expanded and at the same time the graphite is expanded by the gas, and dispersing, the graphite in a surfactant such as TBA to produce graphene. The exfoliation-reintercalation-expansion method also has a low yield of graphene and does not exhibit satisfactory electric properties due to a high interlayer resistance according to the surfactant.
A chemical vapor deposition method involves using transition metals, which easily form a carbon and carbide alloy or properly adsorb carbon at high temperature, as a catalytic layer to synthesize graphene. This method is a complicated process, uses a heavy metal catalyst and entails many limitations for mass production.
An epitaxial synthesis method utilizes a principle that carbon adsorbed on and included in the crystal is growing into graphene along the texture of the substrate surface at high temperature. Graphene manufactured by this synthesis method is very expensive and it is terribly difficult to fabricate a device, as well as it has a lower electric property than that grown by the mechanical exfoliation and chemical vapor deposition methods.
In particular, regarding the method for producing graphene using the above chemical vapor deposition, there is Korea Patent No. 923304 entitled “Graphene Sheets and Method of Producing the Same”. This Patent proposed a method for optimizing a chemical vapor deposition process among Graphene Production Methods. However, as this method uses a catalyst (a catalyst for graphitization), it entails a process for removing the catalyst by acid treatment and a complicated process for producing graphene.
Also, disclosed is U.S. Patent Application Pub. No. 2010/0047154 A1 entitled “Method for preparing graphene ribbons” which is a method for mass production of ribbons comprising: cutting graphite into a short form and permeating water into the crumbled graphite and freezing and expanding the water containing graphite. This description describes a method for production of graphene where graphite is physically cut and graphene is produced using a tensile stress due to a volume expansion while water is frozen. Ultrasonic wave treatment and hydrophilic treatment are required. There is a limitation that not graphene sheets but simply graphene pieces in ribbon forms are obtainable.